Thyroid Dysfunction and Thyroxine-Dependent Programming of Photoinduced Ovarian Growth in American Tree Sparrows (Spizella arborea)

Abstract

The time course of thyroid dysfunction after injecting 0.30 mCi Na131I was charted in female American tree sparrows (Spizella arborea) transferred to long days on Day 0 and challenged with 30 μg bovine thyroid-stimulating hormone on Postinjection Days 4, 7, 46, and 105. Serum T4(L-thyroxine) was not detectable in thyroidectomized birds, indicating complete thyroid dysfunction by Day 4 and no restoration of thyroid function by Day 105. By contrast, serum T4concentrations in similarly challenged thyroid-intact controls greatly exceeded assay sensitivity. To determine whether a single injection of T4can program thyroidectomizedfemaletree sparrows for seasonal reproduction and postnuptial molt, as it can thyroidectomizedmaletree sparrows, thyroidectomized females were injected with T4(100 μg) or alkaline vehicle (V) eitheron the first dayof photostimulation or1 week beforephotostimulation (during which time injected T4likely was cleared metabolically). Females injected with T4on the first dayof photostimulation showed robust ovarian growth similar to that reported for photostimulated euthyroid females. Both groups of V-injected females, as well as females injected with T41 week beforephotostimulation, showed slow, but reliable, ovarian growth, indicating that thyroidectomized birds can detect an increase in day length. When birds were moved at Week 7.5 or 9 to constant light and given T4in drinking water (a qualitative assay for absolute photorefractoriness), only one tested photorefractory and molted. The remaining birds, including those injected with T41 week beforephotostimulation and both groups of V-injected controls, tested photosensitive and did not molt. Taken together, these results indicate that T4, or one of its metabolites, programs thyroidectomized female tree sparrows for ovarian growth early during photostimulation. A single T4injection (100 μg) on the first day of photostimulation usually also programs thyroidectomized male tree sparrows for photorefractoriness and molt. Such an injection is far less effective in programming females for these two late-season events.